A correlator is used in a receiver in an SSC system, e.g. receiver indicated in FIG. 1 in U.S. Pat. No. 4,691,326. FIG. 10 shows the construction of a general asynchronous type 4-bit judgment digital matched filter serving as such a correlator, etc., in which reference numeral 1 and 2 are multipliers; 3 is a carrier oscillator; 4 is a 90.degree. phase shifter; 5 and 6 are low pass filters (LPF); 7 and 8 are A/D converters; 9 and 10 are correlators; 11 and 12 are fixed type weighting circuits; 13, 14 and 17 are adders; and 15 and 16 are squaring circuits. As indicated in the figure, when an asynchronous type digital matched filter receives a signal, it divides received wave into two parts and thus separates the received signal into two signals, called I channel and Q channel, by multiplying signals having the same frequency as the carrier of the received signal, whose phases are shifted by 90.degree. from each other, by the two parts of the received wave in the multipliers 1 and 2, respectively, to convert them in frequency into a base band. Thereafter the two signals are A/D-converted by the A/D converters 7 and 8 through the low pass filters 5 and 6, respectively, to transform the received signal into multi-bit signals. Then, in order to obtain a correlation value, each of the bit signals is correlated with reference data by each of the correlators 9. Thereafter, for the I channel, different bits of correlation values thus obtained are weighted by different weighting circuit 11 and added together by the adder 13. The same operations are effected also for the Q channel. The correlation values thus obtained for the I channel and the Q channel are squared by the squaring circuits 15 and 16, respectively, and added to each other by the adder 17 to obtain a final correlation value.
(c) in FIG. 11 indicates the waveform of the received wave at a point A in FIG. 10, which is a synthesized wave of a desired wave (a) and a non-desired wave (b) in FIG. 11 and has an amplitude corresponding to one of digital values from 0000 to 1111.
Further (a) in FIG. 12 indicates waveforms of different bits B.sub.0 to B.sub.3 at a point B in FIG. 10; (b) in the same figure indicates waveforms of correlation values C.sub.0 to C.sub.3 of the different correlators 0 to 3 at a point C in FIG. 10; and (c) in the same figure indicates a waveform of the correlation value obtained by the addition at a point D in FIG. 10.
However, by the prior art method described above, although an ideal correlation value can be obtained, when a desired wave signal is sufficiently great with respect to other disturbance, D/U increases, when the source transmitting the desired wave is far, when a number of SSCs exist simultaneously etc., and most of desired wave information is not always included in significant bits after it has been decomposed into different bits by an A/D converter. Therefore it had a drawback that, in such a case, electric power should be controlled so that transmission electric power of a desired wave transmitter is increased.